I have hundreds of rats that breed for me and I promise, trying to keep a rat completely healthy is near impossible. Every rat is born with a disease (I apologize for not remembering the name of it right now). If you notice, even since birth, every rat that is ever born will always "sneeze." You could keep a rat on paper towels and completely away from any dust or air born bacteria, and it will in fact, still "sneeze." That is due to the disease that every rat is born with. Now when you get into larger amounts of rats, you'll have multiple rats die every week. Its sad, and never fun to throw them away, but its the truth. You could have them in the perfect conditions, always fed, always watered, n they're always going to die. For no apparent reason 90% of the time. Its just the ways things are when it comes to rats.